E30 Zone

My coolant levels are just about at the middle seam of the plastic reservoir; no higher or near to the two marks halfway up the top section. Coolant warning light came on but the engine seems the have enough and isn't overheating.

Should the coolant be topped to the upper marks and what might be the cause of a non-obvious coolant leak? I've had the head gasket, bolts water pump and cambelt done recently so I'm struggling to understand why there might be a leak.

Is this about an E30? If so, you've posted in the wrong section.
E30 M20 coolant level should be at the horizontal seam, but in my experience, they tend to settle at about 10mm below this level. As long as there's any coolant at all in the bottle the engine should not overheat.
Coolant level sensor faults are usually caused by the wire fracturing right where it goes into the plug of the sensor, or by the sensor itself. If coolant gets inside the stem of the sensor it will rot through the very small wires of the leaf switch inside.
If you buy a new one, be aware that most BMWs that use this type of sensor have the sensor screwed into the underside of the tank. Get the wrong one, and it will turn the light on when the tank is full, and off when it's empty.

Thanks for the response Brian. Yes, it is an E30 and that's really good info and I'll check out the thin, vintage wiring and sensor. Level wise it doesn't seem to have dipped below the seam but I've not checked when cold. My guess is it's fine though and as you say, the culprit for the light lies in the electrics.

Apologies I posted in the wrong section too. Happy for the admins to shift over to where it should be and maintain this lovely new sites zen.